1. Field of the Invention
The present invention relates to an electrophotographic photoconductor used for image forming apparatuses such as an electrophotographic printer, a PPF, and a digital copier.
2. Description of the Related Art
Conventionally, a positively chargeable electrophotographic photoconductor has been demanded, because of its stable discharge and reduced ozone generation during a corona discharge. There are two types of electrophotographic photoconductors, i.e., a multi-layer photoconductor and a single-layer photoconductor.
A multi-layer positively chargeable electrophotographic photoconductor includes a charge generating layer containing a charge generating material and a charge transporting layer containing a charge transporting material. In a conventional multi-layer negatively chargeable electrophotographic photoconductor, the charge transporting layer is formed on the charge generating layer, while in the multi-layer positively chargeable electrophotographic photoconductor, the charge generating layer is formed on the charge transporting layer, since the charge transporting material is mainly a hole transporting material. However, the multi-layer positively chargeable electrophotographic photoconductor is difficult to produce and has not been put to practical use due to the following problems: When the charge generating layer is coated onto the charge transporting layer, the charge transporting material is eluted into the charge generating layer; and the charge generating material is scraped off from the charge generating layer due to the abrasion caused by the repetition of copying. In addition, the multilayer electrophotographic photoconductor essentially has a drawback that a high .gamma. characteristic cannot be obtained.
On the other hand, the single-layer positively chargeable electrophotographic photoconductor has been extensively developed, since it can be more easily produced than the above-mentioned multi-layer positive chargeable electrophotographic photoconductor.
An example of the single-layer positively chargeable electrophotographic photoconductor is disclosed in Japanese Laid-Open Patent Publication No. 1-169454. This electrophotographic photoconductor has a feature in that the photoconductor uses phthalocyanine as a photoconductive material, but does not use the charge transporting material. Because of this feature, such a single-layer positively chargeable electrophotographic photoconductor (hereinafter referred to as a single-layer electrophotographic photoconductor A) has a high .gamma. characteristic and is suitable for a printer, drawing much attention in recent years.
However, the single-layer electrophotographic photoconductor A has the following problems: Since the single-layer electrophotographic photoconductor A does not use the charge transporting material, the sensitivity thereof is lower than that of the functionally divided multi-layer electrophotographic photoconductor using both the charge generating material and the charge transporting material. Moreover, the charge generating material such as phthalocyanine is likely to adsorb gas; therefore, the chargeability of the single-layer electrophotographic photoconductor A is decreased by the influence of active gas such as ozone and nitrogen oxides.
Here, the .gamma. characteristic which is important for the electrophotographic photoconductor will be described.
In order to obtain satisfactory reproduction and resolution of a minute image such as a letter, a developing method having high development sensitivity (i.e., a high .gamma. value). The term ".gamma.", as being used in the present specification, has its origins in silver photography. In the silver photography, .gamma. is represented by the formula: .gamma.=dD/dLogE, where D is an exposure density, and E is an exposure amount. That is, the .gamma. characteristic is the measure of image contrast at different levels of exposure.
In electrophotography, the photoconductor plays a role similar to a film in silver photography. In the electrophotographic process, the voltage at a given point on the surface of an electrophotographic photoconductor which has been uniformly charged will be determined by the amount of light energy that the point has been exposed to. During the development step, this voltage level on the surface of the photoconductor will determine the density of that point on the printed output. The .gamma. characteristic of the electrophotographic photoconductor is defined as the absolute value of a slope of the characteristic curve obtained by plotting the relationship between a surface potential and an exposure energy (the surface potential is plotted on the ordinate and the exposure energy is plotted on the abscissa). This slope is varied depending on the structure and properties of a photosensitive layer. A high .gamma. characteristic according to the present invention corresponds to a large absolute value for the slope of the characteristic curve, where the electrophotographic photoconductor does not respond to small exposure energy of incident light, maintains the surface potential and rapidly responds to exposure energy which has reached a certain degree. In the case where the surface potential is rapidly decreased in a narrow range of exposure energy, the .gamma. characteristic is considered to be high. As the .gamma. characteristic is higher, a minute image can be more clearly reproduced and the resolution thereof is more excellent.
Sensitivity is referred to as a surface potential value at which the surface potential becomes stable at a time when the exposure energy is increased to a certain degree.
Referring to FIG. 1, the features of the multi-layer electrophotographic photoconductor and the single-layer electrophotographic photoconductor A will be described.
FIG. 1 is a graph showing the relationship between the exposure energy (E) and the surface potential (V) in the multi-layer electrophotographic photoconductor and the single-layer electrophotographic photoconductor A.
In this graph, a curve 1 represents an exposure energy (E)-surface potential (V) curve (hereinafter, referred to as an EV characteristic curve) of the single-layer electrophotographic photoconductor A, and a curve 2 represents an EV characteristic curve of the multi-layer electrophotographic photoconductor. As shown in FIG. 1, the sensitivity of the multi-layer electrophotographic photoconductor is high, but the .gamma. characteristic thereof is not high. On the other hand, the .gamma. characteristic of the single-layer electrophotographic photoconductor A is high, but the sensitivity thereof is low.
In order to overcome the above-mentioned problems, the inventors studied a single-layer positively chargeable electrophotographic photoconductor (hereinafter referred to as a single-layer electrophotographic photoconductor B) in which a charge generating material and a charge transporting material are uniformly dispersed in a photosensitive layer, by varying the content of these materials. As a result of the study, the sensitivity of the electrophotographic photoconductor has improved, however, the electrophotographic photoconductor lacks the ability of retaining an electrical potential without responding in the case where the exposure energy is small. Thus, an electrophotographic photoconductor having sufficiently high .gamma. characteristic has not been obtained. The reasons for this have not been made clear. It can be considered that a so-called avalanche phenomenon (described later) is not obtained in the case where the charge generating material and the charge transporting material are dispersed in the photosensitive layer.